1. Field of the Invention
The present invention relates to a protective film for a lower polarizing plate, which is to be bonded to a polarizer to form a lower polarizing plate for a liquid crystal display panel. In particular, the present invention relates to a protective film capable of changing the travel direction of light which has entered a liquid crystal display panel. The present invention also relates to a lower polarizing plate including the protective film, and to a liquid crystal display panel and a display device which include the lower polarizing plate.
Further, the present invention relates a method for producing a protective film for a lower polarizing plate, which is to be bonded to a polarizer to form a lower polarizing plate for a liquid crystal display panel. In particular, the present invention relates to a method for producing a protective film capable of changing the travel direction of light which has entered a liquid crystal display panel. The present invention further relates to a method for producing a lower polarizing plate for a liquid crystal display panel, in particular a lower polarizing plate capable of changing the travel direction of light which has entered a liquid crystal display panel.
2. Description of Related Art
A display device comprising a liquid crystal display panel and a surface light source device for illuminating the liquid crystal display panel from a back side, is in widespread use these days.
The surface light source device includes a light source and a number of optical sheets for changing the travel direction of light from the light source. As in the exemplary surface light source device 120 shown in FIG. 14, the optical sheets in most cases include a diffuser plate 28 and a light diffusing sheet 29 for diffusing light from the light source to obscure the image of the light source, and light condensing sheets 30, 35 which have a function (light condensing function) to condense light into the front direction so as to enhance the front-direction luminance.
On the other hand, the liquid crystal display panel includes a liquid crystal cell capable of controlling the orientation of a liquid crystal for each pixel, a lower polarizing plate disposed on the light entrance side (light incident side, light inputting side) of the liquid crystal cell, and an upper polarizing plate disposed on the light exit side (light outgoing side, light emerging side) of the liquid crystal cell. The pair of polarizing plates includes a polarizer which allows transmission of light of a particular polarization component and absorbs light of a component other than the particular polarization component, and a protective film bonded to the polarizer for protecting the polarizer.
In general, the protective film is comprised of a mere light-transmissive film due to cost constraints and does not positively exert an optical effect on transmitted light. As described e.g. in JP 9-258013A, there is a known protective film to which an optical function is imparted, but only to the extent that the surface of the protective film, on the side not facing a polarizer, is matted because of the constraints of the adhesion to the polarizer and the polarizer-protecting function. The entire contents of JP 9-258013A are incorporated herein by reference.
However, a sufficient diffusing function cannot be imparted to a protective film merely by matting the one surface of the protective film. On the other hand, the freedom of design of the luminance characteristics and the viewing angle characteristics of a display device can be significantly increased if a function to positively change the travel direction of light can be imparted to a protective film. Furthermore, the use of such a protective film could reduce the number of optical sheets incorporated into a surface light source device, making it possible to provide a thinner surface light source device and display device in addition to reducing the production cost.